1. Field of the Invention
The present invention relates to a mobile telecommunications technology, and more particularly, to a random access scheme for a user equipment. Although the present invention is suitable for a wide scope of applications, it is particularly suitable for efficiently processing a random access response message when performing random access in a mobile telecommunications terminal.
2. Discussion of the Related Art
As an example of a mobile telecommunications system to which the present invention may be applied, a 3rd generation partnership project long term evolution (hereinafter referred to as “LTE”) (3GPP LTE) telecommunications system will now be broadly described.
FIG. 1 illustrates a general view of an E-UMTS network structure as an example of a mobile telecommunications system. Herein, the evolved universal mobile telecommunications system (E-UMTS) corresponds to a system evolved from the conventional universal mobile telecommunications system (UMTS). The 3GPP is presently carrying out a basic standardization process for the E-UMTS. Generally, the E-UMTS may also be referred to as an LTE system.
The E-UMTS network may be broadly divided into an evolved-UMTS terrestrial radio access network (E-UTRAN) 101 and a core network (CN) 102. The E-UTRAN 101 consists of a user equipment (hereinafter referred to as a “UE”) 103, a base station (hereinafter referred to as an “eNode B” or an “eNB”) 104, and an access gateway (hereinafter referred to as an “AG”) 105, which is located at an end of the network. The AG 105 may be divided into a portion for processing user traffic and a portion for processing control traffic. At this point, a new interface may be used between a new AG for processing the user traffic and an AG for processing control traffic, thereby enabling the AGs to communicate to and from one another.
At least one or more cells may exist in a single eNode B. An interface for user traffic or control traffic may be used between each eNode B. CN 102 may be configured of a node used for user registration of AG 105 and other UEs 103. Additionally, an interface for differentiating E-UTRAN 101 from CN 102 may also be used.
Layers of a radio interface protocol between a user equipment (or terminal) and a network may be divided into an L1 (i.e., a first layer), an L2 a second layer), and an L3 a third layer), based upon 3 lower layers of an open system interconnection (OSI) reference model, which is generally and broadly known in a telecommunications system. Herein, a physical layer belonging to the first layer provides an information transfer service using a physical channel. Also, a radio resource control (hereinafter referred to as “RRC”) layer located in the third layer performs a function of controlling radio source between the terminal and the network. For this, the RRC layer enables the user equipment and the network to exchange RRC messages to and from one another. The RRC layer may be dispersed in network nodes, such as the eNode B 104 and the AG 105, or the RRS layer may be located only in either one of the eNode B 104 and the AG 105.
FIG. 2 and FIG. 3 respectively illustrate a structure of a radio interface protocol between a user equipment (or terminal), which is configured based upon a 3GPP radio access network standard, and a UTRAN. The radio interface protocol of FIG. 2 and FIG. 3 is horizontally configured of a physical layer, a data link layer, and a network layer, and the radio interface protocol of FIG. 2 and FIG. 3 is vertically divided into a user plane and a control plane. Herein, the user place is used for transmitting data information, and the control plane is used for delivering control signals (or for control signaling). More specifically, FIG. 2 illustrates each layer of the radio protocol control plane, and FIG. 3 illustrates each layer of the radio protocol user plane. As described above, the protocol layers of FIG. 2 and FIG. 3 may be divided into an L1 a first layer), an L2 (i.e., a second layer), and an L3 (i.e., a third layer), based upon 3 lower layers of an open system interconnection (OSI) reference model, which is generally and broadly known in a telecommunications system.
Hereinafter, each layer of the radio protocol control plane shown in FIG. 2 and the radio protocol user plane shown in FIG. 3 will now be described in detail.
A physical (PHY) layer, which corresponds to the first layer, uses a physical channel to provide an information transfer service to its higher layer (or upper layer). The PHY layer is connected to a medium access control (MAC) layer, which corresponds to the higher layer of the PHY layer, through a transport channel. And, data are transported (or transmitted) to and from the MAC layer and the PHY layer through the transport channel. At this point, depending upon the sharing of the channel, the transport channel may be broadly divided into a dedicated transport channel and a common transport channel. Furthermore, data are transported (or transmitted) to and from different PHY layers, i.e., to and from the PHY layer of a transmitting system and the PHY layer of a receiving system, through a physical channel by using a radio source.
Multiple layers exist in the second layer. A medium access control (MAC) layer maps various logical channels to various transport channels. And, the MAC layer also performs logical channel multiplexing, wherein multiple logical channel are mapped to a single transport channel. The MAC layer is connected to its higher layer (or upper layer), a radio link control (RLC) layer, through a logical channel. And, depending upon the type of information that are being transported, the logical channel may be broadly divided into a control channel, which transports information of a control plane, and a traffic channel, which transports information of a user plane.
The radio link control (RLC) layer of the second layer performs segmentation and concatenation on the data received from its higher layer, thereby adjusting the size of the data so that its lower layer can adequately transport the processed data to a radio section. Also, in order to ensure diverse quality of service (QOS) requested by each radio bearer (RB), the RLC layer provides three different operation modes, a transparent mode (TM), an un-acknowledged mode (UM), and an acknowledged mode (AM). Particularly, the AM RLC performs a re-transport function through an automatic repeat and request (ARQ) function in order to transport (or transmit) reliable data.
A packet data convergence protocol (PDCP) layer of the second layer performs a header compression function that reduces the size of an IP packet header, which has a relatively large data size and carries (or contains) unnecessary control information, in order to efficiently transport IP packets in a radio section having a small bandwidth, when transporting IP packets, such as IPv4 or IPv6. By allowing only the information absolutely necessary in the header portion of the corresponding data to be transported, the header compression function enhances the transport efficiency of the radio section. Furthermore, in the LTE system, the PDCP layer also performs a security function. Herein, the security function consists of ciphering and integrity protection. More specifically, ciphering prevents data monitoring (or data surveillance) by a third party, and integrity protection prevents data altering by a third party.
A radio resource control (RRC) layer of the third layer, which corresponds to the uppermost layer in the third layer, is defined only in the control plane. Being associated to the configuration, re-configuration, and release of radio bearers (RBs), the RRC layer controls logical channels, transport channels, and physical channels. Herein, the RB signifies a logical path provided by the first and second layers of a radio protocol, in order to deliver data between the user equipment and the UTRAN. Generally, the configuration of an RS refers to a process of regulating the characteristics of a radio protocol layer and channel, which are required for providing a specific service, and of respectively configuring each specific parameter and operating method. The RB is then divided into signaling RB (SRB) and data RB (DRB). Herein, the SRB is used as a path for transporting an RRC message from the control plane (C-plane), and the DRB is used as a path for transporting user data from the user plane (U-plane).
Downlink transport channels transporting (or transmitting) data from the network to the user equipment include a broadcast channel (BCH) and a downlink shared channel (SCH). More specifically, the BCH transports system information, and the downlink SCH transports other user traffic or control messages. A downlink multicast or a traffic or control message may either be transported through the downlink SCH or may be transported through a separate downlink multicast channel (MCH). Meanwhile, uplink transport channels transporting data from the user equipment to the network include a random access channel (RACH) and an uplink shared channel (SCH). More specifically, the RACH transports initial control messages, and the uplink SCH transports other user traffic or control messages.
Additionally, downlink physical channels transporting information, which are transported to the downlink transport channel, to the radio section between the network and the user equipment include a physical broadcast channel (PBCH), a physical multicast channel (PMCH), a physical downlink shared channel (PDSCH), and a physical downlink control channel (PDCCH) (or a downlink (DL) L1/L2 control channel). More specifically, the PBCH transports information of the BCH, and the PMCH transports information of the MCH. The PDSCH transports information of the PCH and the downlink SCH. And, the PDCCH transports control information provided from the first layer and the second layer, such as a downlink or uplink (DL/UL) scheduling grant. Meanwhile, uplink physical channels transporting information, which are transported to the uplink transport channel, to the radio section between the network and the user equipment include a physical uplink shared channel (PUSCH), a physical random access channel (PRACH), and a physical uplink control channel (PUCCH). More specifically, the PUSCH transports information of the uplink SCH, and the PRACH transports information of the RACH. Furthermore, the PUCCH transports control information provided from the first layer and the second layer, such as an HARQ ACK or NACK, a scheduling request (SR), and a channel quality indicator (CQI) report.
Hereinafter, based upon the above description, the method for performing a random access from the user equipment to a base station (or an eNode B or eNB) will be described in detail. Firstly, the user equipment performs a random access process (or procedure) under the following circumstances:                when the user equipment performs an initial access, due to an absence of an RRC connection between the user equipment and the eNode B        when the user equipment performs a first access to a target cell, during a handover process        when a random access process is requested by a command from the eNode B        when data that are to be transported through an uplink are generated, in case time synchronization of the uplink does not match, or in case a designated radio source is not allocated, the designated radio source being used for requesting a radio source        when performing a recovery process, in case of a radio link failure or a handover failure        
In the LTE system, during the procedure of selecting a random access preamble, a contention based random access procedure, wherein the user equipment randomly selects and uses a preamble from a specific group, and a non-contention based random access procedure, which uses a random access preamble allocated from the base station (or eNode B) only to a specific user equipment, are both provided. However, the non-contention based random access procedure may be used only during the handover procedure (or process) or only upon request from the base station (or eNode B).
Meanwhile, the process of the user equipment performing a random access with a specific base station (or eNode B) may broadly include the steps of (1) having the user equipment transport an random access preamble to the eNode B (or base station) (also referred as a “message 1” transporting step, in case there is no confusion hereinafter), (2) receiving a random access response from the eNode B with respect to the transported random access preamble (also referred as a “message 2” receiving step, in case there is no confusion hereinafter), (3) transporting an uplink message from the random access response message by using the received information (also referred as a “message 3” transporting step, in case there is no confusion hereinafter), and (4) receiving a message corresponding to the uplink message from the eNode B (also referred as a “message 4” receiving step, in case there is no confusion hereinafter).
In the above-described random access procedure, the user equipment stores data that are to be transported through message 3 in a message 3 buffer (or Msg3 buffer). Then, the user equipment transports (or transmits) the data stored in the message 3 buffer with respect to the reception of an uplink grant (or UL grant) signal. The UL grant signal corresponds to a signal notifying information on an uplink radio source, which may be used when the user equipment transports a signal to the base station (or eNode B). Herein, in case of the above-described LTE system, the UL grant signal is received through a random access response (RAR) message, which is received through the physical downlink control channel (PDCCH) or the physical uplink shared channel (PUSCH). Hereinafter, the method for receiving a random access response message of the user equipment will be described in more detail.
FIG. 4 illustrates a method for receiving and processing a random access response message according to a current LTE standard. After transporting the random access preamble, the user equipment attempts to receive its own random access response from within a random access response reception window, which is designated by the base station (or eNode B) through a system information or handover command. More specifically, the random access response information may be transported in a MAC packet data unit (MAC PDU) format. And, the MAC PDU for transporting random access response information includes a MAC payload and a MAC subheader respective of the MAC payload. Herein, the MAC payload corresponds to random access response message information for at least one or more user equipments. The MAC PDU may further include a MAC subheader including a backoff indicator, which may be used when the user equipment reattempts random access. The MAC PDU may be transported through the physical downlink shared channel (PDSCH). Accordingly, in step 601, it is determined whether or not a received random access response message exists within the predetermined random access response reception window. If it is determined that a received random access response message does not exist within the predetermined random access response reception window, it is concluded (or determined) that the reception of the random access response message has failed. Subsequently, the procedure moves on to step 604, so that the operations according to the failure of receiving the random access response message can be performed.
Alternatively, if it is determined that a received random access response message exists within the predetermined random access response reception window, the system determines, in step 602, whether each of the random access response messages received within the random access response reception window includes a random access identifier (e.g., RA-RNTI), which does not correspond to (or match) the random access preamble already transported from the user equipment. If it is determined that all of the random access response messages received within the random access response reception window include a random access identifier, which does not correspond to (or match) the random access preamble already transported from the user equipment, the user equipment concludes that the reception of the respective random access response message has failed. Thereafter, the procedure moves on to step 604, so that the operations according to the failure of receiving the random access response message can be performed. On the other hand, if it is determined that at least one or more random access response messages received within the random access response reception window include a random access identifier, which corresponds to the random access preamble already transported from the user equipment, the procedure moves on to step 603, so that the corresponding random access response message can be processed.